Daylight will peep through a small hole.

The image suggests that a narrow gap is enough for daylight to become visible. What appears closed off may still have an opening through which something can be seen.

Figuratively, a small sign may reveal something someone intended to conceal. A secret or scheme need not be exposed all at once for the first indication of it to matter.

Use the proverb when a minor detail draws attention to something hidden, such as an inconsistency that raises questions about a concealed plan. It emphasizes the revealing power of the clue, not proof that the whole matter is known.